Transconductance mixers may be used in downconverters, upconverters, or other applications which require the multiplication of two signals. They exploit non-linear characteristics of active devices, such as field effect transistors (FETs). In the case of a downconverter, a transconductance mixer generates an intermediate frequency (IF) signal from a local oscillator (LO) signal and a radio frequency (RF) signal. Although not limited to use at any particular frequency range, they are particularly useful when operating at microwave frequencies.
Conversion gain or loss describes the ratio of the IF power to the RF power achieved by a mixer. Compared to diode mixers, which operate at a conversion loss, transconductance mixers operate at a relatively high conversion gain, often times in the 6-8 dB range. However, even higher conversion gains from transconductance mixers would be extremely useful. Generally speaking, high conversion gains are desirable, particularly in battery powered radios, because amplification stages may be omitted. By omitting amplification, more reliable, less complicated, and less expensive radio designs result, and such radio designs use the scarce resource of battery-supplied energy more efficiently.
Transconductance mixers simultaneously meet a plurality of diverse design criteria in order to achieve useful mixing. For example, any spurious signals near the IF range should be effectively eliminated from the input of the active device lest they become amplified and corrupt the desired IF signal. Likewise, LO and RF signals at the output of the active device should be effectively eliminated to prevent instability in the operation of the active device and to prevent degradation of the IF signal. Furthermore, sources of RF and LO signals should be impedance matched to the active device and the active device should be impedance matched to any output circuits which receive the IF signal. Any impedance mismatches result in a reduction of overall conversion gain.
Unfortunately, conventional transconductance mixers fail to satisfactorily meet these design criteria. Typically, in order to achieve stability while effectively eliminating IF range signals at the active device input and effectively eliminating RF and LO range signals at the active device output, impedance mismatches are tolerated. Consequently, overall conversion gain is reduced. Moreover, due to extreme high impedance levels typically presented at the output of the active device, matching networks which are suited to realization in microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs) cannot be used. Matching networks used in conventional transconductance mixers typically require physically large inductors which are not MIC or MMIC compatible. Thus, such matching networks are implemented using discrete lumped elements at an increase in cost, circuit complexity, manufacturing errors, and unreliability.